The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 100 51 508.8, filed on Oct. 18, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a process and to related devices for reducing the ignition voltage of plasmas operated using pulses of pulsed power with a long pulse-off time. Such plasmas are used for treating substrates in, e.g., atomization processes which utilize vacuum coating devices, in plasma activation as part of various coating processes, and in other processes utilizing vacuum technology. Cathode atomization processes, also known as sputtering, represent a primary area of application for pulsed plasmas within the coating technology. With their aid, many things such as components, tools, partially finished products, as well as finished products which are used in optics and mechanical engineering can be coated. Many parts, such as those used in the packaging, glass, and electronics industries are also coated with individual layers or layer systems being deposited on a respective substrate.
2. Discussion of Background Information
In deposited layer systems of this type, bonding agent layers and functional layers with layer thicknesses of only a few nanometers are usually required. In order to deposit layers with such low layer thicknesses, the feed rate of the substrate during the coating process can usually be selected as being set relatively high, in order to shorten the coating duration, or the lag time of the substrate in the coating zone can be kept low, or the electrical power applied can be set very low, which leads to a low depositing of material on the targets of the atomization cathodes, and to a low growth rate of the layers to be deposited.
The substrate speed or the lag time has often been determined by the depositing rates of other layers present in the layer system or to be deposited subsequently. Because of the required stability of the coating plasma, an arbitrary reduction of the electrical power applied is not possible because if the power density is too low, it causes inhomogeneities in the coating plasma and therefore layer inhomogeneities and instability of the coating process. This is particularly problematic in the case of larger cathode arrangements. In practice, stable coating plasmas with a low power feed are presently possible only by using blinds and/or by covering of the sputtering cathodes. This, however, results in a considerable loss of effectiveness.
The bonding agent and functional layers mentioned above are pivotal in determining the properties of the entire layer system, based on their physical properties. Thus, it is important to deposit very thin layers, with precisely defined properties, having a high degree of evenness and reproducibility of these properties, in order to achieve certain target parameters of the entire layer system.
Research has shown that, with the aid of high power densities on the target, the properties of deposited layers can be influenced and, optionally, improved, i.e., Kouznetsov et al., xe2x80x9cA Novel Pulsed Magnetron Technique Using Very High Target Power Densities,xe2x80x9d Surface and Coatings Technology, Vol. 122, 1999, pages 290-293, the disclosure of which is expressly incorporated by reference in its entirety.
Thus, it is often physically necessary to deposit very thin and/or slowly growing layers on the target, as well as using high power densities.
Devices are already known that, with the aid of a pulsed power supply, produce pulses with high electric pulse power utilizing, at the same time, a low average introduced electric power, e.g., DE 41 27 317 A1, the disclosure of which is expressly incorporated by reference in its entirety. The result is a high plasma density on the sputtering target during the pulse-on time. This stabilizes the sputtering plasma at low average electric power levels. However, at the time of coating, a high plasma density is constantly available on the target. The deciding disadvantage of this arrangement lies in the fact that, due to pulse-off times during re-ignition of the discharge on the target, a high ignition voltage results. The value of the ignition voltage depends primarily on the ratio of the pulse-on time to the pulse-off time and the length of the pulse-off time. If the pulse-off times are increased and the pulse-on times are decreased, a very high plasma density on the sputtering target will indeed be achieved at the same average electric power; however, as a result of the long pulse-off time the ignition voltage increases to values that lie substantially above the burning voltage of the plasma and may amount to several kilovolts. This can again cause damage in the various switching arrangements and undesired arcs. Such a result is caused by the fact that, during the long pulse-off time, i.e., at pulse-off times of greater than approximately 1 xcexcs, a sputtering plasma becomes depleted of charge carriers and, as a result, may go out. At the beginning of each pulse, the production of charge carriers must occur again until enough charge carriers are present that ensure that the initially excessive ignition voltage is decreased to the value of the actual burning voltage of the sputtering plasma.
The problem described above generally occurs in plasmas that are pulsed at medium frequency. Wherever the pulse duty factor, i.e., the ratio between pulse-on and pulse off times, is set in such a way that the pulse-off time increases above a critical value or when a longer pulse-off time is necessary for another reason, the critical excesses in ignition voltage may occur.
The invention provides for a process for operating pulsed plasmas which, independently of the pulse-off time, causes an increase in ignition voltage that is as low as possible relative to the burning voltage of the plasma. The process should be suitable in various devices for various vacuum technology processes.
The invention provides for a process for reducing an ignition voltage of power pulses in plasmas operated in a pulsed manner with long pulse-off times, the process comprising generating a power pulsed plasma at a pulse duty factor, the pulse duty factor being a ratio of pulse-on time to pulse-off time, and at least before the beginning of the pulse-on time of the power pulses, producing charge carriers using an additional plasma discharge at a lower power than that of the power pulses.
The charge carriers may be produced by the additional plasma discharge during the entire pulse-off time of the power pulses. The charge carriers may be produced by the additional plasma discharge during part of the pulse-off time of the power pulses. The charge carriers may be produced by the additional plasma discharge partially during the pulse-on time of the power pulses. The charge carriers may be produced by the additional plasma discharge during the entire pulse-off time and pulse-on time of the power pulses. The charge carriers may be produced in a pulsed manner by the additional plasma discharge. The process may further comprise feeding the additional plasma discharge using a separate energy supply device. The process may further comprise regulating a power density of the additional plasma discharge.
The invention also provides for a device for reducing an ignition voltage of power pulses in plasmas operated in a pulsed manner, the device comprising a vacuum chamber or vessel including at least one pump system. An arrangement for producing plasma is provided and the arrangement includes at least one anode and at least one cathode. An energy source is connected to the cathode and the anode which can be pulsed in the frequency range of between approximately 10 Hz to approximately 1 MHz. A mechanism for one of producing an additional plasma discharge is also provided.
The mechanism for producing the additional plasma discharge may be adapted to regulate the additional plasma discharge. The mechanism for producing the additional plasma discharge may comprise at least one device for producing an RF discharge. The mechanism for producing the additional plasma discharge may comprise at least one device for the producing a microwave discharge in the GHz frequency range.
The invention further provides a device for producing a pulsed plasma comprising a vacuum chamber or vessel including at least one pump system. An arrangement for producing plasma is provided and the arrangement includes at least one anode and at least one cathode. An energy source is connected to the cathode and the anode which can be pulsed in at frequency range. A mechanism for one of producing an additional plasma discharge is provided. An ignition voltage of power pulses in the plasma utilizes long pulse-off times. The pulsed plasma is generated at a pulse duty factor, the pulse duty factor being a ratio of pulse-on time to pulse-off time, wherein, at least before the beginning of the pulse-on time of the power pulses, charge carriers are produced using the additional plasma discharge at a lower power than that of the power pulses.
The invention also contemplates a process for producing a pulsed plasma in a device which includes a vacuum chamber or vessel including at least one pump system, an arrangement for producing plasma which includes at least one anode and at least one cathode, an energy source connected to the cathode and the anode which can be pulsed in at frequency range, a mechanism for one of producing an additional plasma discharge, wherein an ignition voltage of power pulses in the plasma utilizing long pulse-off times, the process comprising generating the pulsed plasma at a pulse duty factor, the pulse duty factor being a ratio of pulse-on time to pulse-off time, and at least before the beginning of the pulse-on time of the power pulses, producing charge carriers using the additional plasma discharge at a lower power than that of the power pulses.
The invention is based on the realization that only a fraction of the charge carrier density of technological plasma is necessary in order to effectively prevent highly excessive ignition voltages. If, during the pulse-off times, the plasma is prevented from completely going out or the charge carrier density in the discharge chamber is prevented from decreasing to below a certain low amount, an ignition voltage will occur at the beginning of the following pulse which is only slightly increased relative to the burning voltage of the plasma. The technological purpose of a long pulse-out time, such as, e.g., to guarantee an increased pulse performance with a lower overall average power of the plasma discharge or to prevent defective operating states by using prolonged recovery times, and to prevent additional pulse pauses, is achieved in many cases in spite of the additional charge carrier density.
The process can, in particular, be used for the purpose of producing medium frequency pulsed sputtering plasmas in depositing very thin layers, which leads to a distinct improvement in layer properties. For this purpose, a pulsed energy supply may be connected to a cathode that contains a target made of the material to be sputtered and an anode. The direct current voltage source is characterized in that it operates in the frequency range of between approximately 10 Hz to approximately 1 MHz at possible ratios of pulse-on time to pulse-off time of between approximately 1:1 to approximately 1:100 and above. Thus, it is guaranteed that, at a low average introduced electric power, a high pulse power is produced in the pulse-on time of the discharge.
A second discharge is combined with and/or overlapped with the first such that sufficient charge carriers are produced in the pulse-off time. The task of the second discharge does not lie in sustaining another sputtering plasma and/or a stable discharge for sputtering the target, but rather in maintaining only a certain minimum density of charge carriers during the pulse-off time for the first discharge, which is used for coating. The charge carriers produced in this manner serve to alter the ignition process at the beginning of the pulse-on time of the discharge used for coating, in such a way that the ignition voltage is significantly reduced because of the charge carriers that are present and, in the most favorable case, the burning voltage is approximated. The result is that even very long pulse-off times and very short pulse-on times, which are associated therewith in the exemplary case, can be used. This allows an increase in the power density on the sputtering target at the same average introduced electric power, without the power supply being burdened with a highly excessive ignition voltage.
According to the invention, the ignition voltage may be decoupled from the parameter of the pulse-off time. In such a case, it would depend only on the density of the charge carriers, which are provided by the second discharge.
It is also possible to operate the second discharge in a regulated manner in such a way that the ignition voltage of the power pulses is adjusted to a predetermined value. This is advantageous if, on the one hand, a particular value of the ignition voltage may not be exceeded and, on the other hand, if as low a plasma density as possible is to be present between the individual power pulses.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.